1. Field of the Invention
The present invention relates to a magnetic indication sheet printing method and a magnetic indication sheet printing apparatus for printing characters and such on a magnetic indication sheet on which an image can be magnetically recorded and from which an image can be magnetically erased.
2. Description of the Prior Art
A magnetic indication sheet, i.e., a magnetic indication medium, on which optional information can be magnetically written and from which recorded information can be magnetically erased has been proposed. This magnetic indication sheet is used independently or in combination with a card to indicate information by characters, patterns and such. Magnetic indication sheets disclosed in JP-B No. 48-41221 and JP-A No. 5-16578 are provided with innumerable microcapsules filled up with a suspension prepared by dispersing magnetic microflakes susceptible to a magnetic field in a transparent liquid. When a vertical magnetic field is applied to such a magnetic indication sheet, the magnetic microflakes are aligned to enable light to penetrate into the back surface of the magnetic indication sheet, so that the color of portions of the back surface corresponding to aligned magnetic microflakes become visible to indicate characters, patterns and such by color contrast. The aligned magnetic microflakes can be turned by a horizontal magnetic field to conceal the color of the back surface. Accordingly, the magnetic indication sheet can be used as a writable or erasable indication means.
Methods of writing information on or erasing information from a magnetic indication sheet are disclosed in JP-A Nos. 4-17089, 5-12049 and 55-12011.
Incidentally, the viscosity of the suspension filling up the microcapsules of the magnetic indication sheet is dependent on temperature. At a low temperature, the suspension has a high viscosity and hence the magnetic microflakes are dull of behavior in response to the applied magnetic field; consequently, characters, patterns and such cannot be indicated in a high contrast to the background and print quality is deteriorated, because the magnetic microflakes reflect light to obscure the color of the back surface if the magnetic microflakes are held in an oblique position. The viscosity of the suspension is low at a high temperature and the magnetic microflakes respond sharply to a magnetic field and are very sensitive to a slight leakage flux, which reduces the resolution and the contrast of magnetic indication and deteriorates print quality. Such temperature-dependent problems occur because each magnetic indication sheet has an optimum printing temperature. Thus, the magnetic indication sheet can be used in a narrow working temperature range and, in some cases, the magnetic indication sheet cannot be used at temperatures lower than the optimum working temperature.